Magnetic field sensing devices which detect rotation and/or position have been widely used. Rotation and/or position is detected by detecting changes in the strength of the magnetic field. Magnetic field strength is defined as the magnetomotive force developed by a permanent magnet per unit distance in the magnetization direction.
For example, an increase in the strength of magnetic field, corresponding to the drop in the reluctance of a magnetic circuit, will occur as an object made from a high magnetic permeability material such as iron is moved toward the magnet. Magnetic permeability is the ease with which the magnetic lines of force, designated as magnetic flux, can pass through a substance magnetized with a given magnetizing force. Quantitatively, magnetic permeability is expressed as the ratio between the magnetic flux density (the number of lines of magnetic flux per unit area which are perpendicular to the direction of the flux) produced and the magnetic field strength, or magnetizing force.
The output signal of a magnetic field sensing device is dependent upon the strength of the magnetic field. It is therefore effective in detecting the distance between the sensing device and an object within the magnetic circuit. The range within which the object can be detected is limited by the flux density, as measured in gauss or teslas. It should be mentioned that steel conducts magnetic flux much more greatly than air. It thus has a lower "reluctance" to the passage of magnetic flux than air.
It may be desired to determine the speed or rotation position of a low reluctance rotating object, such as a steel disk mounted on a shaft. In such instance, the steel disk is typically provided with distinctive circumferential surface features, such as one or more teeth that project outward from the circumference of the disk toward the sensing device. As the disk rotates on its axis, the proximity of a tooth to the sensing device will increase the strength of the magnetic field in the spatial region of the sensing device. Accordingly, by monitoring the output of the sensing device, the rotation or speed of the disk can be determined by correlating the peaks in the sensor's output with known number of teeth on the circumference of the disk. Likewise, when the teeth are irregularly spaced in a predetermined pattern, the rotation or position of the body can be determined by correlating the peak intervals with the known intervals between the teeth on the disk.
Two prominent forms of such sensing devices are the magnetoresistor and the Hall effect sensor. A magnetoresistor is a device whose resistance varies with the strength of the magnetic field applied to the device. Generally, the magnetoresistor is a slab of semiconductive material. For many automotive applications, the preferred form of a magnetoresistor is a thin elongated body of a carrier mobility semiconductor material having a high current carrier mobility, such as indium antimonide (InSb) or indium arsenide (InAs). The thin elongated body has electrical contacts at its ends. It is recognized that the body does not have to be rectangular. It can have a multiplicity of shapes, including sinuous and/or U-shaped. If U-shaped, the contacts will, of course, be adjacent one another. In any event, the magnetoresistor is mounted within and perpendicular to the flux in a magnetic circuit which includes a permanent magnet and an exciter. The exciter is a high magnetic permeability element having special edge or surface conformations projecting toward the sensor. These projecting surface features increase the strength of the magnet's magnetic field between the surface of the exciter and the permanent magnet. As indicated above, the exciter can be in the form of a wheel or disk having a series of circumferentially spaced teeth, such as a gear. Alternatively, it can be a disk having one or more radial slots in its circumference that define gaps in its periphery. When the exciter disk rotates, its outer periphery moves linearly toward and away from the stationary magnetoresistor element. In doing so, it changes the reluctance of the magnetic circuit involving the magnetic field sensor. This causes the magnetic flux through the magnetoresistor element to vary in a manner corresponding to the position of the teeth and/or the slots on the exciter disk. With the change in magnetic flux, there occurs a corresponding change in magnetic field strength to which the magnetic field sensor is exposed. If the magnetic field sensor is a magnetoresistor, higher magnetic field strength increases resistance, and lower strength reduces resistance. One can view a magnetoresistor as a voltage device if the current flow is small.
A Hall effect sensor is also a semiconductor device and is packaged analogous to the packaging of a magnetoresistor. However, a Hall effect device relies upon a transverse current flow that occurs in the presence of a magnetic field. Accordingly, a Hall effect sensor is primarily a current device. As mentioned, a magnetoresistor is a voltage device. The Hall effect sensor is primarily driven by a direct current source having electrodes contacting opposite ends of the Hall effect sensor. This creates a longitudinal current flow to the sensor's body. In the presence of a magnetic field, a transverse current is induced in the sensor, which can be detected by a second pair of electrodes transverse to the first pair, i.e., on opposite sides of the body. The second pair of electrodes can be connected to a volt meter to determine the electrical potential transversely created across the surface of the sensor by the magnetic field. Variations in the magnetic field will create variations in the transverse potential across the surface of the sensor. Accordingly, the transverse potential in a Hall effect sensor is analogous to the voltage output of a magnetoresistor and can be analogously used for magnetic field detection.
With increasing sophistication of products, magnetic field sensing devices have also become common in products that rely on electronics and their operation, such as automobile control systems. Common examples of automotive applications where magnetic field sensing can be used include detection of engine crank shaft and/or cam shaft rotation and position for ignition timing. It also includes detection of wheel speed for antilock braking systems and for four-wheel steering systems. Other sensing applications are also of interest. For detecting wheel speed, the exciter is typically a high permeability metal wheel mounted inboard from the vehicle's road wheel. The exciter wheel can be coaxially mounted with the road wheel so as to rotate at the same speed as the road wheel. The exciter wheel typically will have a number of teeth which extend radially from the perimeter of the exciter wheel toward a magnetic field sensor. The teeth can be on an inner or outer circumference of the exciter wheel. As noted before, the exciter wheel is formed of a high magnetic permeability material, such as iron or steel. As a tooth on the exciter wheel rotates toward the magnetic field sensor, the strength of the magnetic field increases in the spatial area of the sensor as a result of a decrease in the magnetic circuit's reluctance in that spatial area. Subsequently, the magnetic circuit reluctance increases and the strength of magnetic field decreases as the tooth moves away from the magnetic field sensor. The result is an increase in voltage across a magnetoresistor and a transverse current increase in a Hall effect device.
A well-known shortcoming of magnetic field sensing devices is their critical dependence upon distance between the exciter and the sensing device itself. This distance is generally referred to as air gap. It commonly refers to the distance between the exciter and the magnetic field sensing semiconductor chip surface, even if part of that distance includes a protective plastic encapsulation around the semiconductive magnetic field sensing chip. Magnetic field strength decreases in air as a function of the squared distance from the source. Accordingly, as air gap increases, the tooth/slot output voltage difference of the magnetic field sensor decreases. As this difference decreases, the more difficult it is to accurately recognize that the output of the device has varied.
The output of a magnetoresistor is particularly susceptible to the detrimental effects of a large air gap in relatively low strength magnetic fields, such as magnetic fields found in typical automotive applications (approximately 500 to 2000 gauss). As indicated above, this is detrimental because resistance of the magnetoresistor is dependent upon the square of the magnetic field's strength. Specifically, the resistance of the magnetoresistor under the influence of a magnetic field is a direct function of mobility squared times this magnetic field strength squared.
Conventionally, the air gap is defined as the distance between the exciter and the outer surface of the package containing the sensing device. An "effective air gap" may be described as the distance between the exciter and the active surface of the active component in the sensing device. A semiconductor chip forms the active element in a magnetoresistor or Hall effect device. In commercially available devices, a relatively thick plastic coating environmentally and mechanically protects the chips and also provides or reinforces terminal connections to the semiconductor chips. The plastic encapsulation frequently also encloses a permanent magnet associated with the semiconductor magnetic field sensing chip. For particularly harsh environments, such as automotive environments, the encapsulated chip often has to be repackaged. It is repackaged in still an additional housing that also covers the active area of the chip. This provides additional environmental protection. However, it also interposes still another layer of plastic between the exciter and the semiconductor chip itself. This further increases the "effective air gap". Thus, while improving the life of the sensing device, a particularly significant shortcoming to this "repackaging" approach is that it decreases the change in magnetic field strength as a tooth passes close to the sensing device. This is due to the larger "effective air gap".
U.S. Pat. No. 5,250,925, issued Oct. 5, 1993 to George A. Shinkle, discloses an improved packaging technique by which the originally encapsulated chip can be retained in the holder without adding another layer of plastic between the exciter, i.e., the toothed wheel, and the originally encapsulated chip. It does so by providing a hollow housing that can receive the originally encapsulated semiconductor chip and magnet, and has means for sealing the leads extending from that encapsulation within the housing. In a preferred embodiment, the prepackaged unit is cylindrical and has a circumferential groove, within which an o-ring seats. The housing has a cylindrical recess within which the prepackage unit nests, with the o-ring providing an interfacial seal.
Maintaining minimum "effective air gap" is an important factor in using a magnetic field sensor. Accurately positioning the magnetically sensitive semiconductor chip with respect to the exciter wheel is also important, especially if the exciter wheel is quite thin. If not positioned directly in line, "effective air gap" increases, and sensitivity thereby decreases. Positioning is particularly important if multiple sensors are used in a single package. This invention makes such positioning much easier. In addition, lower cost manufacturing techniques are desired. This invention helps in this latter connection as well. Still further, as electronic products are miniaturized, the sensor packages have to be miniaturized. This invention helps here too. In addition, new concepts in packaging are desired for new types of magnetic field sensing applications. This invention permits use of magnetic field sensing in new ways.